Wideband bio-imaging system and related methods

ABSTRACT

An imaging and recordation system is provided. The system includes a high-power, focusing antenna for illuminating biological tissue. The system further includes a power source for powering the antenna. The system further includes a data acquisition module, for recording the dielectric properties of tissues illuminated by the high-power, focusing antenna. The system illuminates the tissues using ultrashort electrical pulses.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a §371 national stage entry of InternationalApplication No. PCT/US08/87754, filed Dec. 19, 2008, which claimspriority to U.S. Provisional Patent Application No. 61/016,363, filedDec. 21, 2007, the entire contents of which are incorporated herein byreference.

FIELD OF THE INVENTION

The present invention is related to the field of imaging, and moreparticularly, to systems and methods for imaging using electromagneticwaves.

BACKGROUND OF THE INVENTION

Microwave imaging can be used to determine the dielectric permittivitydistribution of objects using measurements of a scattered electricfield. In a biological context, such imaging of biological tissueshaving different dielectric permittivities results in differentscattering cross sections. The contrast obtained with this imaging isgenerally greater than that obtained with xray radiography, which is onthe order of a few percent. Since the electrical conductivity anddielectric constant of tumors, for example, are an order of magnitudehigher than normal tissue over a wide frequency range, typically in therange of 10⁷-10¹⁰ Hz, tumors can be readily differentiated from thenormal tissue. Moreover, with such imaging irregular physiologicalchanges resulting from the deterioration of health can also bemonitored.

Conventional microwave imaging is predominantly based on narrowbandtechnology and records scattered signals at a single frequency, oralternatively, at multiple frequencies recorded at different times. Apulsed microwave imaging system on the other hand, contains a wide rangeof frequencies. For example, a Gaussian-pulse varies in time accordingto the following relation:g(t)∝exp(−t ² /a ²); and  (1)the corresponding frequency range is given byG(f)∝exp(−π² f ² /a ²)  (2)

For a=100 ps, which corresponds to a pulse with a width of 100 ps, thefrequency range can extend to f=5 GHz. Although microwave imaging in thetime domain has been studied, typically only back-scattered signals havebeen utilized, in an approach to detect tumors in breast tissue.

SUMMARY OF THE INVENTION

The present invention is directed to systems and related methods forimaging and real-time recording of the dielectric properties ofbiological tissue. One of the varied aspects of the invention isutilization of ultrashort electrical pulses. The ultrashort electricalpulses, more particularly, can have durations on the order of 100 ps(i.e., more than 10 ps, but less than 100 ps).

BRIEF DESCRIPTION OF THE DRAWINGS

There are shown in the drawings, embodiments which are presentlypreferred. It is expressly noted, however, that the invention is notlimited to the precise arrangements and instrumentalities shown.

FIG. 1 is a schematic view of a system, according to one embodiment ofthe invention.

FIG. 2 is a schematic view of an antenna construct utilized in thesystem of FIG. 1, according to one embodiment of the invention.

FIG. 3 is data representing Gaussian input impulse with respect to time.

FIG. 4 is a schematic illustration of 3-D scanning of tissue by changingthe focal point of the antenna by (a) adjusting the position of theantenna or (b) by adjusting the position of the patient.

FIG. 5 is a schematic illustration of a confocal imaging system.

FIG. 6 is a schematic illustration of a confocal microwave imagingsystem.

FIG. 7 (a) depicts the configuration of the antenna, tissue and tumor;7(b) depicts the focusing at the second focal point; 7(c) depicts thevoltage measure at the cone apex relative to the ground.

DETAILED DESCRIPTION

The invention is directed to systems and methods for recording spatialand temporal changes in the electrical properties of tissues located ina human or an animal using ultrashort electrical pulses. The ultrashortelectrical pulses, more particularly, can have durations on the order of100 ps.

FIG. 1 is a schematic view of an imaging and real-time recordationsystem 100, according to one embodiment of the invention. The system 100illustratively includes a high-power, focusing antenna 102 forilluminating tissues, a power source 104, and a data acquisition module106. The focusing antenna, different embodiments of which are describedmore particularly below, is used to focus the radiation into the tissue.A focal point is then moved such that an entire volume of interest isscanned. The antenna 102 can produce a high electrical field pulse (>10kV/cm at the focal point). This high power capability can achieve a highsignal-noise ratio and facilitate detection of both backward scattering(reflection) and forward scattering (transmission), even if the signalis strongly attenuated in the tissue.

In one embodiment, the system 100 uses 100 ps pulses. Using 100 pspulses, considering the diffraction-limit (λ/2), a resolution of a fewmillimeters can be obtained. The focusing ability of the system 100allows 3-D scanning and filtering. The data acquisition module 106 cancomprise fast-response electric field sensors 108. According to oneembodiment, one or more of the electric field sensors 108 comprise ACD Ddot sensors manufactured by Prodyn, which can record the free spaceelectric field with a rise time of 50 ps.

The data acquisition 106 can further include one or more processors 110.The one or more processors 110 can be configured to execute, indedicated circuitry and/or computer-readable code, image reconstructionalgorithms in 3D FDTD inverse scattering, such as a gradient-basedminimization algorithm.

According to one embodiment, the antenna 102 comprises an antenna systemthat includes a conical wave launching system and a reflector. The wavelaunching system 200 has rotational symmetry about the z-axis, asillustrated schematically in FIG. 2. The apex of the cone is at thefirst focal point F1. So, a wave emitted from F1 is a spherical TEM wavewith its source at F1. The apex is connected to the inner conductor of acoaxial cable feed from one of the ends of the reflector. Such conicalwave launcher above a ground plane is an unbalanced structure; thus, nobalun is required to feed the wave launcher.

The launched wave is reflected on the prolate spheroidal reflectorsurface to converge at the second focal point (F2). The electric fieldat F2 consists of prepulse, impulse and postpulse. The prepulse, causedby the diffraction at the edge of the wave launcher cone has the samepolarity as the impulse, the scattered signal from the reflector. Due torotational symmetry, the electric field at the second focal point hasonly longitudinal components.

In an alternative embodiment, the antenna 102 comprises a configurationthat includes symmetrical conical arm feedings (either 4 or 2 arms) thatfrom the first focal point to the reflector. A balun is used to suppressthe common mode signal. The focused electrical field has its largestcomponent in transverse direction.

Based on modeling results, using FDTD software Magic, the focusingpattern has an oval shape. Its size is 2-3 cm along the z axis and lessthan 1 cm along x axis for Full Width at Half Maximum (FWHM). The sizeof the focusing spot can be much smaller for smaller changes in signalamplitude. For example, recording changes at 90% of its peak increasesthe resolution to mm.

The focusing spot size typically does not fully determine the resolutionof the imaging. The resolution is also determined by the quality of thecollected scattered signal (e.g., the signal to noise ratio (SNR) andthe quantity of the data, etc.) and the image reconstruction algorithmused in the inverse scattering process. Using nonlinear algorithms, itcan be shown that the spatial resolution of the image can be improved tovalues of 1/7 or 1/10 of the wavelength, which is also referred to asthe super-resolution.

The power source 104 comprises a high power pulse source that feeds theantenna 102 with durations on the order of 50 ps-150 ps. This can beproduced by a Marx generator charging a peaking capacitor with a peakingspark gap switch. Capacitors in the generator are displaced in a coaxialwaveguide to minimize the stray inductance. High pressure gas is neededto minimize the peaking spark gap switch distance at a high hold-offvoltage. Output voltages up to 500 kV, 100 ps can be generated from suchgenerator. The resulting focused field at the second focal point is onthe order of 10-250 kV/cm. Other fast transient pulse source can bebased on an inductive energy storage discharged through a solid-stateopening switch (SOS). In both approaches, high frequency operation of >1kHz is possible.

Signal attenuation in the human body can result in a low signal/noiseratio. For example, in normal breast tissue, the attenuation is close to4 dB/cm for frequencies up to 10 GHz. The attenuation is due to Ohmicloss, but for impulses that cover a large frequency range, dispersionlosses must be included. The variation of the index of refraction (orpermittivity) with frequency, causes broadening of the pulse.

The complex permittivity of tissue can be described by a Debye model, inwhich only one relaxation time constant is taking into account, orCole-Cole model, in which multiple relaxation time constants areincluded.

The propagation factor γ(f) isγ(f)=j2πf√∈ _(r) /c  (3)where ∈_(r) is the dielectric permittivity of the medium and c is thespeed of light in the vacuum. The distorted temporal pulse can beretrieved using an inverse Fourier transform:v(t,z)=iFFT{FFT[v(t,0)]exp(−γ(f)·z)}  (4)

For example, in water, which is a major constituent in the body, thedispersion can be calculated for a Gaussian input impulse with rise time(10-90%) of 140 ps. After the pulse travels 0.2 m and 0.4 m, the peakmagnitude decreases to 30% and 20% respectively. Moreover, the pulsewidth broadens and the risetime increases to 370 ps and 480. FIG. 3depicts this data.

A signal with a high amplitude is likely to alleviate problems ofattenuation and dispersion. For example, a 100 kV/cm pulse allows areduction of its signal by 50 dB, and still generates a signal in therange of a few V/cm. Using a high power radiation system, which radiatesan impulse in a very short exposure time (on the order of 100 ps),allows the probing of deeper lying tissue despite high attenuation.

According to another embodiment of the invention, an imaging sequence isprovided, which is implemented according to the following steps: 1)focusing the radiation into a site, where a tumor or other abnormalityin permittivity could be located (at the focal point, tissue scattersthe radiation into 4π (a full sphere), generally through Miescattering); 2) recording both back scattered signal and forwardscattered signal; and 3) processing data generated as a result andreconstructing 3-D images.

The focusing capability of the radiation system allows scanning of thetissue in three dimensions. As the focal point of the radiation antennachanges, the scattered signal from the tissue contains the informationof the distribution of the complex dielectric permittivity in 3-D.Scanning can be done either by adjusting the position of patient or theposition of the antenna as shown by system 400 in FIGS. 4 a and 4 b.

The focusing capability of the antenna allows the realization of theconfocal microwave imaging. A confocal imaging system can be usedgenerally to record fluorescence. A laser beam with one wavelength willbe focused at the focal point illuminating the object to excite the dyemolecules, which fluoresce at a longer wavelength. Confocal technologytypically only allows the fluorescence from the focal point to berecorded through the object lens because the pinhole works as a spatialfilter, which can block the fluorescence from the spots that are out offocus as sown in the confocal imaging system 500 of FIG. 5 by the focalpoint.

Microwave confocal imaging can be realized through a focusing radiationsystem. The scattered signal at the focal point where tumor tissue islocated is omni-directional. Part of the scattered waves will be actingas a secondary source and propagate back to the emitter. In this case,the conical wave launcher typically works not only as a wave radiatorbut also a sensor to record the backward scattering signal. Only thescattered signal from the focal point will be collected, whereasscattered signals from other objects will arrive at the first focalpoint at a different time. Such a concept can be found in FDTD modelingas “time gating,” which can filter unwanted signals by reading thesignals at different times. Thus, such radiation structure permits theradiation to be focused into the potential site, and also serves tofilter other unwanted signals. This is depicted in one embodiment in theconfocal microwave imaging system 600 of FIG. 6.

The underlying concept can be verified by modeling, wherein the voltageis modeled at the junction of the cone apex and the coax cable feeding.As this junction is the conjugate of the second focal point, and thesignal is collected. For example, using a tumor (1×1 cm) with adielectric constant of 100 in a fat layer (10), the antenna was filledwith a coupling medium which has the same dielectric constant as fat.

FIGS. 7( a)-(c) illustrate the configuration of the antenna, tissue andtumor; the focusing at the second focal point; and the voltage measureat the cone apex relative to the ground. The voltage measured at thewave launcher can differentiate the signals in the tumor and non-tumorcase at t=6.3 ns. This time is equal to the round trip time (2r, where ris the major radius of the prolate spheroidal reflector) from F1 to F2.This method consequently provides increased resolution.

Because of the high amplitude signal of the radiation and theconsequently strong signal of the scattered signal, not only thebackward scattered signal (which is the case with confocal imaging) canbe recorded but also the forward scattered signal. This provides moreinformation than a conventional approach, which only covers thereflected signal. Accordingly, the time for the image reconstruction tobe reduced dramatically.

A conventional approach can be used for image reconstruction: aninverse-scattering algorithm. In the time domain, a FDTD for solving theforward scattering problem combined with optimization process can beused. By constructing a cost function, which includes all the trialvalues for the tissue property, a minimization process can be iteratedto find all the tissue properties and form 3-D images. Because theimaging system can record the scattering in any direction, the imagereconstruction process can provide fast and highly precise results.

Ultra-short pulses, (for 100 ps, lie amplitude, 5 GHz), can beconsidered as nonionizing radiation, which is significantly safer thanionizing radiation (i.e., x-rays used in mammography.) Also, thermaleffects can be substantially neglected. Non-thermal effects, forexample, electric field induced electroporation, were not observed for alarge number of 800 ps pulses up to a few thousand shots.

This instrument can be used to detect changes of dielectric propertiesof tissues (i.e. the permittivity, and the conductivity), which can becaused by structural and functional changes of the tissues. Tumors, forexample, which have a significantly higher dielectric constant andconductivities than normal tissues, can be detected and followed overtime.

The deviation of the dielectric properties of tissues from their normalstates, for example, caused by external stimulations, can be monitored.For example, the function of heart, can be monitored by detecting thevariations of the dielectric properties with respect to those for thehealthy tissues. Injuries or illnesses which cause changes in theelectrical properties of tissue can be detected and monitored.

In one embodiment, the pulses used in the imaging of the exemplaryembodiments can be of high intensity. For example, using the antennasystems of the exemplary embodiments, it is possible to generateelectric fields of more than about 100 kV/cm in tissue. This can allowfor the exploration of tissue aberrations in deeper lying tissue, suchas in liver and other organs. Due to the high intensity of the pulse,the scattered signals, which can be used to evaluate the tissue, canstill have sufficient amplitude to be recorded by sensitive sensors, inspite of large losses in intensity the pulse may suffer when it passesthrough the tissue to reach the target and from the target to thesensor. This is in contrast to other techniques that suffer from usingsmall signals, which allow to probe only targets close to the bodysurface, or targets, where the absorption is small, such as in breasttissue.

The invention, as already noted, can be realized in hardware, software,or a combination of hardware and software. The invention can be realizedin a centralized fashion in one computer system, or in a distributedfashion where different elements are spread across severalinterconnected computer systems. Any kind of computer system or otherapparatus adapted for carrying out the methods described herein issuited. A typical combination of hardware and software can be a generalpurpose computer system with a computer program that, when being loadedand executed, controls the computer system such that it carries out themethods described herein.

The invention, as also already noted, can be embedded in a computerprogram product, which comprises all the features enabling theimplementation of the methods described herein, and which when loaded ina computer system is able to carry out these methods. Computer programin the present context means any expression, in any language, code ornotation, of a set of instructions intended to cause a system having aninformation processing capability to perform a particular functioneither directly or after either or both of the following: a) conversionto another language, code or notation; b) reproduction in a differentmaterial form.

The foregoing description of preferred embodiments of the invention havebeen presented for the purposes of illustration. The description is notintended to limit the invention to the precise form that is disclosed.Indeed, modifications and variations will be readily apparent from theforegoing description. Accordingly, it is intended that the scope of theinvention not be limited by the detailed description provided herein.

Additional aspects of the invention are described in the inventors'paper, which is included in U.S. Provisional Patent Application61/015,363, filed Dec. 21, 2007, which is hereby incorporated byreference in its entirety.

We claim:
 1. An imaging and recordation system, comprising: ahigh-power, focusing antenna for illuminating a remote, external focalpoint of the high power, focusing antenna; a power source for generatingultrashort electrical pulses for the antenna; and a data acquisitionmodule, for recording the dielectric properties of one or more objectspositioned at the focal point and illuminated by the high-power,focusing antenna; wherein the high power, focusing antenna comprises awave launching system for generating electromagnetic waves based on theultrashort electrical pulses and a focusing reflector associated withthe wave launching system and configured with reflecting surfaces forcausing the electromagnetic waves from the wave launching system toconverge at the focal point, wherein the reflecting surfaces define aprolate spheroidal reflector.
 2. The system of claim 1, wherein theultrashort electrical pulses have a duration on the order of 100 picoseconds (ps).
 3. The system of claim 1, wherein the wave launchingsystem comprises a conical wave launching system.
 4. The system of claim1, wherein the data acquisition module implements an inverse scatteringalgorithm.
 5. The system of claim 1, wherein the power source isconfigured to feed the antenna high-power pulses with durations on theorder of 50-150 pico seconds (ps).
 6. The system of claim 1, furthercomprising an image-reconstruction module.
 7. The system of claim 6,wherein the image-reconstruction module implements an imagereconstruction algorithm used in an inverse scattering process.
 8. Thesystem of claim 7, wherein the image-reconstruction module is configuredto generate a spatial resolution of an image with values of 1/7 or 1/10of a wavelength based on a non-linear algorithm.
 9. The system of claim1, wherein the ultrashort electrical pulses have a duration greater than10 pico seconds (ps) and less than 1000 ps.
 10. The system of claim 1,wherein the power source is configured to feed the antenna high-powerpulses with durations greater than 10 pico seconds (ps) and less than1000 ps.
 11. The system of claim 1, wherein the focusing antenna furthercomprises at least one lens.